Combine the egg whites and sugar in the bowl of a mixer and place it over a pot with one inch of simmering water, gently whisking until the temperature reaches 160 degrees on a candy thermometer, or until the sugar has completely dissolved and the egg whites are hot.
With the whisk attachment, beat the mixture on medium-high for a good ten minutes, or until the bowl has completely cooled down.
Switching to the paddle attachment on low speed, begin adding butter, a couple tabs at a time, and allow to mix for several seconds before adding more. Continue until all the butter is gone and the buttercream is smooth and shiny.
Add the vanilla and salt and beat a few more times.